1. Field of the Invention
The present invention relates to a table feed system for moving and positioning a feed table of a machine tool.
2. Description of the Related Art
Concerning a table feed system for moving and positioning a feed table of a machine tool, a motor-driven type table feed system, in which a servo motor and a ball screw are combined with each other so as to conduct numerical control (NC control), is commonly used at present. In the case where high accuracy, high output and high rigidity are required for cutting and grinding in the operation of a machine tool, it is necessary to use a large-scale, complicated and expensive table feed system. When mirror face work is conducted in the field of precision work, feed marks, which are caused by the feed command, are transferred onto the work face. Therefore, quality of the mirror face is impaired.
On the other hand, concerning the hydraulic type table feed system, although the hydraulic type table feed system is small-scale and the output is high, its structure can be made simple. However, the energy transmission medium used in this system is a liquid such as a hydraulic fluid. Since liquid has compressibility and relatively high viscosity, it is not appropriate as an energy transmission medium to be used in numerical control in which a high response property is generally required. Therefore, at present, the hydraulic type table feed system is not often applied to a system in which highly accurate table feed is required. Especially, the hydraulic type table feed system is seldom used in the table feed system of a machine tool, the operation speed of which is high, in which high accuracy, of the micron order, is required.
However, as one of the few examples in which a hydraulic cylinder is applied to the table feed system of a machine tool for precision work, Japanese Unexamined Patent Publication No. 8-150527 discloses a technique for a hydraulic type table feed system. In this example, as a means for preventing feed marks from being transferred onto a work face in the process of mirror face work, a static pressure bearing for supporting a piston rod is combined with a hydraulic cylinder, and a static pressure coupling is interposed between the piston rod and the feed table. Due to the above structure, the feed table can be smoothly moved, so that the generation of feed marks, which are caused by pulsation of the hydraulic pressure and fluctuation of the piston shaft, can be prevented. However, the piston feed control system is the conventional flow rate control conducted by a proportional valve and others. Therefore, it is possible to realize a smooth feeding motion by this prior art, however, it is impossible to obtain a high positioning accuracy. The prior art has the above problems of controllability.
As the prior art, and to enhance controllability, Japanese Unexamined Patent Publication No. 58-82643 discloses a table feed system in which pneumatic pressure is utilized. According to this technique, a feed table is pulse-driven by ON-OFF valves, which can be easily controlled by a computer, so that a highly accurate feed motion can be accomplished. According to this table feed system, it is possible to enhance the positioning accuracy, however, feed marks appear on a work face because the feeding waveform is step-like.
The present invention has been accomplished to solve the above problems of the prior art by a new means. The first object of the present invention is to provide a table feed system for a machine tool, at low cost, which is capable of smoothly moving a table so that a mirror face can be made by realizing a highly accurate feed motion conducted by a pulse drive of high controllability by using a computer.
Specifically, the second object of the present invention is to extend control ranges of the feed speed and of the resolution by making a flow rate of hydraulic fluid to a hydraulic cylinder, for operating a table feed system, very small.
In the table feed system of the present invention, a pair of hydraulic chambers, one being arranged on the right and the other being arranged on the left, are formed in at least one hydraulic cylinder corresponding to the moving direction, to right and left, of the table feed of a machine tool, and a pressure adjusting mechanism is arranged, by which pressure of hydraulic fluid in each hydraulic chamber can be independently adjusted. The characteristic of this table feed system is described as follows. Pressure in the hydraulic chambers, one being arranged on the right and the other being arranged on the left, is independently increased and decreased by the pressure adjusting mechanism. When the piston is moved to the right and the left by the pressure difference between the hydraulic chambers, a time difference occurs between periods in which the pressure adjusting mechanism conducts an adjustment of pressure on the respective hydraulic chambers.
When a time difference occurs between periods in which the pressure adjusting mechanism conducts an adjustment of pressure on the respective hydraulic chambers as described above, when pressure in one hydraulic chamber is increased, pressure in the other hydraulic chamber is not simultaneously decreased. Therefore, at first, the piston is moved to a position at which pressure in both hydraulic chambers becomes an equilibrium condition. After that, when pressure in the other hydraulic chamber is decreased, the piston is moved to a position at which the decreased pressure in the other hydraulic chamber and pressure in one hydraulic chamber become an equilibrium condition. According to the increase and decrease in pressure in the right and the left hydraulic chamber, which are made with a time difference, the piston is smoothly moved little by little. Accordingly, even when a pressure adjusting mechanism such as a common electromagnetic valve, the minimum opening and closing period of which is long, is used, it is possible to obtain the same effect as that of a case in which the valve is opened for a period of time shorter than the minimum valve opening period. Therefore, it is possible to obtain a high controllability by an inexpensive system of a simple structure. Further, it is possible to finely adjust a flow rate of hydraulic fluid to the hydraulic cylinder by a pressure adjusting mechanism such a common electromagnetic valve. Therefore, highly accurate positioning can be done by a simple structure. Accordingly, ranges of controlling the feed speed and the resolution can be extended.
This table feed system utilizes hydraulic pressure, which has seldom been used in recent years. However, this table feed system can be formed into a digital system when valves composing the pressure adjusting mechanism are subjected to duty control by a stream of pulse signals. Therefore, controllability by the computer is high, and it is possible to provide the same effect as that of a case in which a valve, the minimum valve-opening period of which is very short, is used. Accordingly, a highly accurate feed can be easily realized at low cost. This table feed system utilizes hydraulic pressure and is different from the conventional motor-driven digital system. Therefore, due to a slight compressibility of hydraulic fluid, step portions of displacement are chamfered, so that a smooth motion can be made. Accordingly, in the process of machining or drilling, it is possible to reduce a sudden change in a load given to a cutting tool. Further, the hydraulic system is advantageous in that the damping property is high. Therefore, the hydraulic system can provide an effect that the occurrence of chatter marks is prevented.
This table feed system is characterized in that a time difference is provided between the time at which pressure adjustment is conducted on the right hydraulic chamber and the time at which pressure adjustment is conducted on the left hydraulic chamber. As an embodiment, it is possible to provide a phase difference between the duty control of the hydraulic fluid inflow valve of one hydraulic chamber and the duty control of the hydraulic fluid outflow valve of the other hydraulic chamber. As the most remarkable case, the phases are set to be opposite to each other. Even in the above case in which pulse-like hydraulic fluid flows into one hydraulic chamber and the table is moved in pulses by the pulsed flow of hydraulic fluid, when pulses of hydraulic fluid of the inverse phase is made to flow out from the other hydraulic chamber, the pulses are cancel each other. Therefore, the piston and the feed table can be smoothly and linearly moved and displaced.
When a setting is made in such a manner that the valve opening period in which one of the hydraulic fluid inflow valve and the hydraulic fluid outflow valve, which are arranged in the same hydraulic chamber, is opened for the hydraulic chamber of the hydraulic cylinder partially overlaps the valve opening period in which the other valve is opened for the hydraulic chamber or in such a manner that the valve opening period in which one of the hydraulic fluid inflow valve and the hydraulic fluid outflow valve is opened for the hydraulic chamber of the hydraulic cylinder is set longer than the valve opening period in which the other valve is opened for the hydraulic chamber, the pressure adjustment of the hydraulic chamber is canceled. As a result, the effective valve opening period is a difference between the valve-opening periods of the two valves, that is, the effective valve-opening period is short. For the above reasons, even if a common inexpensive valve, the minimum opening and closing period of which is relatively long, is used for a pressure adjustment mechanism, it is possible to provide a sufficiently high accuracy and response speed.
In order to solve the above problems, the present invention provides the following table feed system as a specific means.
In this table feed system, corresponding to the moving direction of the feed table of a machine tool to the right and left, a pair of hydraulic chambers, one being arranged on the right and the other being arranged on the left, are formed in at least one hydraulic cylinder, and an outflow valve for feeding to the left and an inflow valve for feeding to the right are connected with the left hydraulic chamber, and an inflow valve for feeding to the left and an outflow valve for feeding to the right are connected with the right hydraulic chamber. In this way, the pair of valves is connected with each hydraulic chamber. Further, there is provided a control means for independently controlling the pressure of hydraulic fluid in the pair of hydraulic chambers. When a stream of pulse signals is generated and the four valves are subjected to duty control, the valve can be opened or closed.
Consequently, according to this table feed system, a stream of pulse signals are supplied to the respective four valves by the control means, and the four valves are subjected to duty control so as to be opened and closed. Due to the foregoing, an arbitrary intensity of pressure can be given to the two hydraulic chambers. Since a pressure difference between the two hydraulic chambers can be arbitrarily controlled in this way, the piston and the feed table, which are moved by the pressure difference, can be moved to and stopped at an arbitrary position. Even when a common electromagnetic valve, the minimum opening and closing period of which is long, is used, it is possible to obtain the same effect as that of a case in which an electromagnetic valve, the minimum opening and closing period of which is short, is used for the valves. Therefore, it is possible to obtain high controllability by an inexpensive system of a simple structure. Further, it is possible to easily conduct a highly accurate positioning. Accordingly, the ranges of controlling the feed speed and resolution can be extended.
More particularly, the table feed system of the present invention can be composed as follows. There are provided a pair of hydraulic cylinders, wherein one is arranged on the right and the other is arranged on the left. The forward end portions of the piston rods of the right and the left hydraulic cylinder are made to come into contact with a table drive piece in such a manner that the forward end portions of the piston rods of the right and the left hydraulic cylinder are opposed to each other while the table drive piece connected with the feed table is being interposed between the forward end portions of the piston rods of the right and the left hydraulic cylinder. Due to the above structure, the piston rod of each piston pushes the table drive piece so as to move the feed table. In this case, the piston rod and the table drive piece are not moved in a different direction from the feed direction being linked with each other. Therefore, the piston rod is not given a force, the direction of which is different from the feed direction, by the weight and thermal expansion. The effect can be made certain when the forward end of each piston rod is chamfered so that the piston rod comes into contact with the table drive piece by point contact.
Even if a clearance formed between the hydraulic cylinder and the piston is extremely reduced, the sliding motion of the piston is not prevented. Therefore, it is possible to omit an O-ring made of rubber which is usually arranged to tightly seal this clearance. Since no O-ring is used in this structure, the hydraulic chamber of the hydraulic cylinder has no elasticity. For the above reasons, although it is a hydraulic type table feed system, fast response can be realized in control. When the valve is integrated with the cylinder block into one body and a flow path of hydraulic fluid connecting the valve with the hydraulic chamber in the hydraulic cylinder is formed in the cylinder block, rigidity of the flow path can be enhanced, which becomes a factor to enhance the response property of control. For the same reasons, it is preferable to abolish an O-ring used for tightly sealing a joint of a flow path for connecting the valve with the hydraulic chamber in the hydraulic cylinder. It is preferable to use a metal seal instead of the O-ring in this case.